1. Field of the Invention
The present invention relates to comfort fittings positioned on the interior of boots, and especially sport boots with a rigid external upper, such as the shells for ski boots or skates, and has as an object the use of a foam wedging element having thermoplastic qualities and adapted to adjust and/or to adapt itself, after it has been heated at its thermoforming temperature, to the specific volume of the user's foot.
2. Description of Background and Relevant Information
In known ski boots comprising internal fittings with this type of thermoplastic foam wedging element, the initial fitting volume, i.e., before adjustment by thermoforming, is, for a given size, either smaller than the standard for this size, or considerably larger, or still not defined. By way of example, such boots are described in the patents EP 004 829, FR 2 460 118, JP 2-270519 and in PCT Application WO 94/09663. As is disclosed, none of the internal liners used in these boots is preformed to a standard fitting volume for a given size, and thus cannot be utilized for skiing until after thermoforming. In effect, in the case of the patent EP 004 829, it is a question of giving room for the foot because the initial cavity is undersized by about one to two sizes with respect to the foot to which one wishes to adapt the boot. The process therefore consists of heating the foam of the wedging element by means of an electric heating element, then, after introduction of the foot of the skier, of compressing said wedging element which is sandwiched between the shell and said foot, and of allowing it to cool in this position.
In the case of the patents FR 2 460 118 and JP 2-270519, it is the opposite operation which is performed. The internal liners are obtained preformed with thermocompressed walls, thus having a relatively high density in this state, and it is by heating them that one causes more or less their relaxation and therefore their adjustment to the foot and within the shell. As this appears clearly, the preforming by thermocompression necessitates providing a cavity, or fitting volume, which is much larger than the volume of the foot to be held because the constituent materials of the walls of the internal fittings having been preliminarily thermocompressed have lost much of their flexibility, and it is impossible to readjust them if desired to a greater volume simply by pressure of the foot.
Such internal liners preformed by thermocompression thus have, for a given size, a considerably greater fitting volume, and their walls in the thermocompressed state have a high density which renders them inappropriate in assuring an acceptable comfort for the foot if they are not heated to relax, and thus in restoring a certain flexibility.
In the example of the internal liner described in PCT Application WO 94/09663, the problem is different from the preceding problems because there, the internal liner is not preformed in its initial state, in fact the fitting volume is not defined; indeed, according to this document, it is essentially due to the integral heating of the liner that it is possible to adapt it on the foot, which, thus equipped, is then introduced in the boot. Therefore, this type of internal liner cannot be, as in the preceding cases, utilized in its initial state for purposes of skiing.
Another disadvantage appears likewise in the internal liners described hereinabove and relates to the stability of the imprints achieved after reheating at the thermoforming temperatures of the materials utilized, such as polyethylene or polyurethane. In effect, these materials which are made in the form of foam are sensitive to repeated pressure and are crushed and collapse with use. Thus, such internal liners must be readjusted to the skier's foot quite often so as to always provide an optimum grip and comfort. So as to limit the number of these readjustment interventions, it is known to vary the density of these foams: a high density providing a high resistance to crushing but a lesser comfort since it is less flexible and less compressible, and conversely, a low density providing a low resistance to crushing but an increased comfort due to the flexibility and substantial compressibility of the foam. The comfort/duration optimum compromise over time is therefore very difficult to achieve.
Furthermore, in the case where the foam is made of polyurethane, a supplemental problem is posed with respect to providing an imprint, because such a foam is not thermoplastic and as a result cannot be put into a specific form or allow for an adjustment on the foot of the skier simply by means of a heat source. To overcome this disadvantage, it is proposed in the patent JP 2-270519 to mix the polyurethane in the form which is given to it, such as the imprint of the foot. The addition of the resin thus confers to the polyurethane foam properties and behavior similar to those of a thermoplastic foam.
During a readjustment to a new foot imprint, it then suffices to reheat the foam charged with resin until the latter becomes plastic to allow the polyurethane to relax and/or to compress itself depending on the form imposed by this new imprint, and to let it cool. This type of internal liner with a polyurethane wedging element loaded with resin proves however relatively uncomfortable because the flexibility and initial compressibility of the polyurethane foam are almost eliminated by the resin which, in fact, is the element which gives the consistency of the foam thus obtained.
This type of problem is not posed with foams made of thermoplastic materials such as polyethylene, ethylene vinyl acetate polymer, and polypropylene for example, because their thermoplastic nature does not require the addition of a resin. However, other disadvantages occur because of their very thermoplastic nature. One concerns their flexibility and compressibility which are relatively inferior than those of polyurethane, which detracts from comfort; and the other, their sealed structure which does not permit a good diffusion of heat throughout their mass during the heating operation to bring them to the temperature which renders them plastic, and therefore thermoformable.